The left anterior descending coronary artery of anesthetized dog hearts will be alternatively occluded or perfused with blood devoid of oxygen in order to differentiate effects of myocardial hypoxia from those of ischemia. The effects of long term (2 hours) hypoxia and subsequent perfusion with normoxic blood will be studied. Measurements will be made of myocardial contractility, metabolites, regional myocardial blood flow, coronary vascular volume, and autoregulatory adjustments to changes in perfusion pressure. Epicardial electrograms will be recoded during non-ischemic myocardial hypoxia and compared with those recorded during ischemia. Myocardial blood flow, metabolism, and contractility will be studied during non-ischemic myocardial hypoxia and compared with responses to ischemia induced by coronary artery occlusion. Samples of myocardium will be analyzed for adenosine concentration to determine if the changes in adenosine concentration correlate with changes in coronary vascular resistance.